--> Abstract: Evaluation of Lateral and Vertical Fluid Migration in the Pyrenees-Macedon Oil and Gas Fields, Australian Northwest Shelf, by Wayne R. Bailey, James Underschultz, Dave Dewhurst, Gillian Kovack, Scott Mildren, Mark Lisk, and Mark Raven; #90039 (2005)

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Evaluation of Lateral and Vertical Fluid Migration in the Pyrenees-Macedon Oil and Gas Fields, Australian Northwest Shelf

Wayne R. Bailey1, James Underschultz1, Dave Dewhurst1, Gillian Kovack2, Scott Mildren3, Mark Lisk1, and Mark Raven4
1 CSIRO Petroleum, Perth, Australia
2 University of Adelaide, Adelaide, Australia
3 JRS Petroleum Research, Adelaide, Australia
4 CSIRO Land and Water, Australia

The Pyrenees-Macedon (P-M) fields, in the Exmouth Sub-basin on the Australian Northwest Shelf, are currently under-filled relative to available closure despite being a regional focal point for Cretaceous to Recent charge. Late structural development of the P-M trap with respect to charge was thought to be the reason for under-filling. However, seismic amplitude anomalies and gas shows above the reservoir suggest across-fault and/or vertical leakage may have controlled column heights. Hydrodynamic analysis of pressure data also suggests that faults separating the fields act as barriers to lateral migration of hydrocarbons and water, whilst faults within the Macedon field do not.

The reasons for hydrocarbon leakage and the difference in fault seal capacities are investigated by integrating structural observations, analysis of pressure and stress data, appraisal of both top (mercury porosimetry measurements) and fault (Shale Gouge Ratios; SGR) membrane seal capacities, geomechanical assessment, and well-based fracture analysis. The top seals are at a low risk of capillary failure, but vertical leakage is possible via dynamic failure along pre-existing faults and conductive fractures. Top seal breach is also possible via lateral, across-fault leakage at reservoir-thief zone fault juxtapositions. The difference in observed fault seal capacities between individual faults can be explained by a combination of the spatial distributions of SGR and buoyancy pressure. The procedure presented delivers a robust description of the key risks concerning reservoir connectivity and the integrity and capacity of seals where fluid migration over long (geological) and short (tectonically related) timescales must be risked.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005